But a simple number that measures someone's temperature is of very little use if the question is whether they should increase or reduce their caloric intake.

That's the situation here.

The human development index composed of average Rank in terms of Purchasing Power Parity GDP per capita, Rank in terms of Life Expectancy, and Rank in terms of Adult Literacy is an example of combining three simple numbers in a quite arbitrary, ad hoc, and certainly imprecise way, which despite that provides a more robust measure of relative standard of living than any of them individually would do.

GDP per capita measures capacity to provide material support to individual needs at most ... but comparing a society that gives a hundred and one people $1,000 a month and a society that gives a hundred people $100 month and one person $91,000 a month would yield the same $12,000/capita.

But you cannot "store" a thousand years of lifespan in one wealthy person to make up for ten years of lifespan lost by a hundred poorer people ... life expectancy track strongly to how well a society distributes its ability to meet basic needs. And adult literacy tracks as well as any readily available number to measure distribution of access to social participation.

Its got a gaping hole in terms of sustainability, but the way to fill that would be to add a sustainability rank, such as the rank in terms of excess (or deficit) biocapacity per capita in hectares.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.